Multiple selection in GridView (asp.net)

In order to get multiple selection in a GridView, you need a field in your datasource that is unique. Then use a checkbox with the unique field to do multiple selection:

<asp:gridview>
<columns>
  <asp:templatefield headertext="Select">
    <itemtemplate>
      <asp:checkbox id="cbSelect" runat="server">
      <asp:label id="uniqueId" runat="server" 
                 text='<%# Eval("uniqueid") %>' 
                 CssClass="hidden" />
    </asp:label>
  </asp:checkbox>
  ...
  </itemtemplate>
</asp:templatefield>
</columns>
</asp:gridview>

You can then loop through the rows in the GridView and see which ones have been selected, picking out the unique ids for those.

The CSS for hidden is simply:

.hidden {
  display: none;
}

Which hides the item, but still renders it in html so that you can retrieve it (unlike Visible=false which does not render it in html at all).